Light-hole transitions in quantum dots: Realizing full control by highly focused optical-vortex beams

An optical vortex is an inhomogeneous light beam having a phase singularity at its axis, where the intensity of the electric and/or magnetic field may vanish. Already well studied are the paraxial beams, which may carry well-defined values of spin (polarization sigma) and orbital angular momenta; the orbital angular momentum per photon is given by the topological charge l times the Planck constant. Here we study the light hole-to-conduction band transitions in a semiconductor quantum dot induced by a highly focused beam originating from a l = 1 paraxial optical vortex. We find that at normal incidence the pulse will produce two distinct types of electron-hole pairs, depending on the relative signs of sigma and l. When sgn(sigma) = sgn(l), the pulse will create electron-hole pairs with band+spin and envelope angular momenta both equal to 1. In contrast, for sgn(sigma) l not equal sgn(l), the electron-hole pairs will have neither band+spin nor envelope angular momenta. A tightly focused optical-vortex beam thus makes possible the creation of pairs that cannot be produced with plane waves at normal incidence. With the addition of co-propagating plane waves or switching techniques to change the charge l both the band+spin and the envelope angular momenta of the pair wave function can be precisely controlled. We discuss possible applications in the field of spintronics that open up.